In this paper, we present an analysis correlating the fitting parameter α in the Arya and Paris (1981) soil water retention model to physical properties of the soil. Fractal mathematics are used to show that α is equal to the fractal dimension of the pore trace and expresses a measure of the tortuosity of the pore trace. The fractal dimension of the particle-size distribution can be easily measured and related to the α parameter of the Arya and Paris model. By suggesting a physical significance of the coefficient, the universality of the model is greatly improved. Soil water retention data, estimated strictly from particle-size distributions, are proven to match measured data quite well. The fractal dimension of pore traces range from 1.011 to 1.485 for all but one soil tested.

Miscible displacement experiments were conducted to describe chromium [Cr(VI)] mobility and interactions in six different soils. For Calciorthid, Webster, and Norwood soils Cr breakthrough curves (BTCs) indicated that the interactions with the soil matrix were similar to those for a nonreactive solute where no or small retardation of the equilibrium type was observed. These observations are in support of earlier kinetic batch results where little retention was observed for a wide range of Cr concentrations in these (high pH) soils. The BTCs from Olivier, Cecil, and Windsor soils indicated high Cr retention capacity as indicated by increased retardation, low peak concentrations, irreversible sorption and extensive effluent tailing during desorption (leaching). A nonlinear retention/release model is proposed for the purpose of describing Cr(VI) reactions during transport in soils. The model is incorporated into the convection-dispersion transport equation for reactive solute in uniform soils. The model was capable of providing a good description of the Cr BTCs for all soils where model parameters were obtained using a nonlinear least squares (best fit) parameter optimization scheme. The model grossly underestimated effluent concentrations for Olivier and Windsor BTCs when independently measured retention/release rate coefficients from the batch data sets were used. Model predictions overestimated the amount irreversibly retained by Cecil soil. Model predictions indicated that a unique set of independently measured rate coefficients was not capable of providing an adequate description of Cr BTCs for these soils.

Approved for publication by the Director of the La. Agric. Exp. Stn. as manuscript no. 88-09-2554.

Soil detachment by raindrops is a precursor to the start of interrill erosion and surface seal formation. Soil detachment occurs when erosive forces of raindrops are greater than the inherent strength of soil. Experiments were conducted to study the importance of inherent soil strength on detachment of soil by raindrops. Sand strength was controlled by varying matric potentials before raindrop impact. A range of raindrop kinetic energies and momentum was created by changing drop diameter (3.6–5.0 mm) and fall height (1–8 m). The experimental setup consisted of a varying height water drop former, an electronic drop discriminator, and a single-drop soil splash collector. Splash collected by varying drop height of a 4.6-mm-diam. drop on sand equilibrated at a matric potential (ψm) of −1.5 kPa (higher strength) showed that a larger threshold energy is needed to initiate soil detachment than when equilibrated at ψm = −0.1 kPa (lower strength). Also, the threshold erosivity was larger in magnitude and more significant when using momentum as an index of erosivity instead of kinetic energy. The relationships between sand splash and erosivity were linear (P < 0.001) for raindrops with diameters of 3.6 to 5 mm falling from heights of 0.76 to 7.6 m on sand equilibrated at ψm = −1.0 kPa. Sand splash was represented by a model of the form D = Kd[E − Eo]b, where D is detachment, E is raindrop erosivity, Eo is threshold erosivity, Kd is sand detachability, and b is an exponent equal to 1. This equation of describing soil detachment as a function of erosivity is an improvement over the currently used nonlinear relationship of the form D = KdEb.

Sediments from six tidally influenced Chesapeake Bay marshes were sequentially extracted in order to determine their relative Fe, Zn, Cu, Pb, Ni and Cd content, and to ascertain the relative magnitude of the mechanisms (e.g. adsorption, sulfide occlusion, organic complexation etc., ..) involved in their retention. The extraction sequence used, and the corresponding geochemical phases extracted were: 0.01 M DTPA (exchangeable, organic complexes); 0.1 M HC1 (monosulfide occluded); dithionite citrate bicarbonate (DCB) (Feoxide occluded); H2O2 (disulfide occluded, organically bound complexes); HF-HNO3-HC104 (silicate mineral component). The sediments were dated using 210Pb geochronology in an effort to evaluate historical rates of metal deposition. Marshes in the vicinity of Baltimore, MD harbor were found to contain relatively higher metal concentrations than those marshes in other areas of the bay thus indicating that the degree of metal contamination is related to the source distance. Rates of deposition were related to historical periods of industrial discharge and to the use of leaded fuels. Iron was retained in the marsh sediments mainly in the oxide and sulfide (FeS2) phases and was apparently controlled by redox conditions. Copper, Cd and to a lesser extent Zn and Ni were apparently controlled by sulfide precipitation and pyrite coprecipitation thus rendering them noninfluential and unavailable to the marsh biota. Lead however, appears to be weakly complexed by organics and is a potential ecological threat since it is more readily available to plants and organisms and thus may accumulate in the food chain.

The objective of this research was to determine the nature and extent of increased soil levels of Cu or Cd on K availability properties. Bozeman silty clay (Typic Argiborolls) samples were equilibrated at soil water potentials of −33 or −100 KPa, at temperatures of 5 or 20 °C for 3 or 14 d after additions of three levels of K and three levels of Cu or Cd. Soil solution was extracted by an immiscible liquid displacement technique. The activity of Ca, Mg, and K in soil solutions were increased by Cu or Cd additions (aCa > aMg > aK). The ratio of K to Ca + Mg (K intensity) was decreased by Cu or Cd additions when no K was applied, but increased at high levels of K addition. Soil temperature and water potential interacted with Cu or Cd additions to alter cation activities. Soil K buffer capacity was decreased by Cu or Cd additions. Increasing addition of K increased the proportion of K in both exchange and solution phases, at the expense of both Ca and Mg. Increasing additions of Cu or Cd decreased the fractional saturation of K and Mg, but increased that of Ca in the solution phase, with the reverse influence being exhibited on the exchange phase. Results indicate that Cu had a stronger influence than Cd (at equal rates of addition) on soil properties which regulate K availability.

The relationship between soil K forms (water soluble, exchangeable, HNO3-extractable, and mineral) as a function of clay mineralogy and taxonomy, was determined using 102 soils from the continental USA and Puerto Rico, representing 10 soil orders. These relationships may have use in evaluating potential soil K fertility and prediction of K cycling and plant uptake. The soils were divided into three groups (kaolinitic, mixed, and smectitic) based on dominant mineralogy of the clay fraction. For each group of soils, water soluble K was related to exchangeable K content (r2 of 0.86 to 0.96), with exchangeable K solubility increasing from smectitic, mixed, to kaolinitic soils. For each group, exchangeable K was also related to HNO3-extractable K (r2 of 0.81 to 0.83), thus, the capacity to supply K under continuous cropping, is greater for smectitic than kaolinitic soils of similar exchangeable K contents. Mineral K was a function of clay content for each soil group (r2 of 0.66 to 0.90). Using these relationships, water soluble (2–37 mg kg−1), HNO3-extractable (21–2691 mg kg−1), and mineral K contents (1.59–16.93 mg kg−1) of 60 independent soils, were closely predicted (r2 of 0.78 to 0.96) from exchangeable K, clay content, and clay mineralogy. These results indicate that determination of both exchangeable and HNO3-extractable K could give a better indication of the potential K supplying power of a soil. Further, the relationships obtained will allow determination of initial soil K pool sizes from readily available soil taxonomic, clay mineralogy, and chemical properties, for use in modeling the cycling and plant uptake of soil K.

Contribution from Agronomy Dep., Oklahoma State Univ. and USDA-ARS, Durant, OK, under a cooperative agreement with Oklahoma State University and USDA-ARS.

Pressure-jump (p-jump) relaxation with conductivity detection was used to ascertain the kinetics and mechanisms of MoO4 adsorption/desorption on goethite. A postulated reaction mechanism consisting of two consecutive elementary steps was examined and verified through kinetic and equilibrium studies. The first step is the formation of an ion-pair complex through the electrostatic attraction between the protonated surface and the MoO4 anion. The second step involves a ligand exchange process, whereby 1 mol of H2O is replaced by 1 mol of adsorbed MoO4 from the surface. It is much slower than the first step. The forward and backward intrinsic rate constants for steps 1 and 2 are: kint1 = 4019.2 mol−1 L s−1, kint-1 = 391.5 s−1, kint2 = 1.888 mol−1 L s−1, and kint-2 = 42.34 s−1. A slightly modified triple layer model (TLM) was employed to calculate the distribution of ionic species on the goethite surface, in the α and β layers, and in the bulk solution at equilibrium and electrical parameters for the charged surface. Both equilibrium and kinetic data fit the postulated mechanism for the reaction steps and the modified adsorption model well.

The Beaumont (fine, montmorillonitic, thermic Entic Pelludert) and Lake Charles (fine, montmorillonitic, thermic Typic Pelludert) soils along the Texas Gulf Coast produce only about 2 Mg rice (Oryza sativa L.) ha−1 without N fertilizer, while the Nada soil (fine, montmorillonitic, thermic Typic Albaqualf) frequently produces 5 Mg ha−1. In studying differences between these soils, data showed that NaCl applied to the Beaumont soil did not reduce rice yield, but equivalent amounts of KCl did. The KCl-induced yield reduction may have been the result of NH+4 entrapment in clay minerals caused by added K. Clay mineral characterization showed that the Beaumont soil fixed more NH+4 than the Nada soil because the Beaumont soil was higher in soil K, high-charge smectite [i.e., 0.76 equivalents per (Si,Al)4O10(OH)2] and charges in the tetrahedral sites. The 8-wk incubation of Beaumont soil in the rice root zone resulted in partial release of added Nh+4 and no release of native NH+4 when the Beaumont soil had been Ca saturated. The K-saturated Beaumont soil did not release fixed Nh+4 during incubation as the Ca-saturated soil did. The Lake Charles soil showed clay and fixation characteristics similar to the Beaumont soil, while the Nada soil did not fix beyond its native level or release any upon incubation. The presence of 2:1 layer silicates in Beaumont and Lake Charles soils with x-ray diffraction characteristics similar to smectite, and NH+4 fixation characteristics similar to vermiculite, was recognized.

The pH-dependent charge properties of the A and B horizons of two representative oxisols from the Cerrado Region, Brazil, were studied by a comparison of the potentiometric titration method of net charge measurement and direct measurement of anion and cation exchange capacity from K+ and Cl- adsorption. Most soil samples bore net negative charge at their native pH, but variability in the magnitude of this charge was attributed to the effect of Al-blocked exchange sites or contributions from strongly acidic organic functional groups, since these soils had similar mineralogies. The soils also bore significant pH-dependent positive charge at their natural acidic pH values, which was more evident in the B horizons than the surface horizons. Estimates of net surface charge by potentiometric titration and ion adsorption did not agree, especially as the soil pH was adjusted away from the point of zero charge. This lack of agreement is attributed to dissolution reactions of minerals and organic matter at high and low pH, which consume acid or base and overestimate surface charge.

Contribution from the Dep. of Agronomy, Cornell Univ. Supported in part by the Tropsoils Project.

The kinetics of Al precipitation/dissolution reactions in Spodosol Bs horizons from sites at Hubbard Brook, NH and Bear Brook, ME were examined. A mechanical vacuum extractor was employed to draw solutions through soil columns at solution/soil residence times between 0.3 and 100 h. Equilibrium was approached from conditions of both undersaturation and oversaturation to determine if a particular soil mineral controlled Al3+ activity. Column leachates were analyzed for major solutes followed by chemical speciation and calculation of mineral saturation indices. Our results showed that apparent equilibrium with respect to Al(OH)3 solubility was readily obtained from both conditions of undersaturation and oversaturation within 0.3 h. Computed saturation indices, for an Al(OH)3 mineral with log K25C = 8.1, were identical at all examined residence times. Aquo Al (Al3+) within soil solutions may be regulated by the hydroxy-Al interlayer of expansible 2:1 layer silicates. Soil solutions from Bear Brook also reached apparent equilibrium with imogolite at residence times in excess of 10 h. The laboratory results compared favorably with saturation indices calculated from stream and soil solutions at these sites.

Phosphorus-31 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to examine the forms of P in two municipal sludges and a sludge-amended soil. The signal-to-noise ratio and the resolution of the spectra improved considerably when paramagnetics in the sludge and sludge-treated soil samples were removed by a reducing treatment involving citrate-bicarbonate-dithionite (CBD). The spectra suggested that the P solid phases in the anaerobically digested sludge from Los Angeles County, California were calcium phosphates. The strong paramagnetic effects, however, precluded more precise identification of the calcium phosphate phase. In the anaerobically digested alum-treated sludge from Riverside, three P solid phases are detected, carbonated apatite, a pyrophosphate, and aluminium phosphate. In the Domino soil (fine-loamy, mixed, thermic Xerollic Calciorthid), amended with sludge from Los Angeles County, 31P MAS NMR indicated the presence of carbonated apatite and pyrophosphate solid phases.

current address: Dep. of Chemistry, Michigan State Univ., East Lansing, MI 48824-1322. Contribution from the Dep. of Soil and Environmental Sciences, Univ. of California, Riverside, and the Division of Chemistry and Chemical Engineering, California Inst. of Technology.

The solubility of P in two sludge-amended soils was studied. Solubility and activity ratio diagrams for P in sludge-amended soils were constructed and were compared with those of selected Ca-P, Fe-P, and Al-P minerals. The solid phase controlling P solubility in the sludge-amended soils appeared to be a carbonated apatite formed through coprecipitation. The 31P magic angle spinning nuclear magnetic resonance (31P MAS NMR) examinations confirmed that P in the sludge-amended soils was in form of calcium phosphate. The resonant peak [at isotropic chemical shift (δlso) = 3.1 ppm] is indicative of an apatitic calcium phosphate environment characterized by carbonate substitution. Additionally, pyrophosphate was detected by the presence of a peak at δlso = −9 ppm.

Contribution from Dep. of Soil and Environmental Sciences, Univ. of California, Riverside.

The negative impact of nitrate (NO3) on groundwater supplies has sparked a great deal of interest and concern in recent years, particularly in areas where coarse-textured soils abound. In light of this concern, the anion exchange chemistry of eight soils from the Middle Atlantic region was studied with particular emphasis on NO3 retention and kinetics. The soils were chosen to encompass a range of physicochemical and mineralogical properties and were extensively characterized. Anion exchange capacity (AEC) was determined on Cl-saturated samples by desorption of Cl with SO4. Anion exchange capacity ranged from 0 to 1.35 cmolc kg−1 for the eight soils and was found to parallel increases in clay and Fe oxide contents in the soil profiles. Point of zero salt effect (PZSE) values were determined by potentiometric titration with 0.001, 0.01, and 0.1 M NaCl as the indifferent electrolyte. These were of little value in predicting the development of AEC for the soils. The kinetics of NO3 adsorption and desorption were studied using a stirred-flow reaction chamber and a first-order reaction best described the data. Nitrate adsorption was found to be completely reversible, indicating a simple electrostatic retention mechanism. The effect of pH and NO3 concentration on cumulative NO3 adsorbed (CNA) and on NO3 adsorption kinetics was also studied. The CNA was found to increase with a decrease in pH below 5.5 and to increase with increasing NO3 concentration. The latter indicated an increase in competitiveness by NO3 for positively charged sites.

Published as Miscellaneous Paper no. 1275 of the Delaware Agric. Exp. Stn. Contribution no. 247 of the Dep. of Plant Science, Univ. of Delaware.

Formation and escape of the gaseous products of aerobic soil N transformation processes such as nitrification are logically studied in the laboratory using an incubation system that neither interferes with the diffusive transport of gases within the soil nor allows accumulation and further reaction of gaseous intermediates after their escape through the soil surface. In addition to satisfying these criteria, our flow-through incubation system provides for continuously monitoring soil emissions of nitric oxide, nitrous oxide, and carbon dioxide, while also permitting soil analysis as frequently as required to characterize changes in the pool sizes and transformation rates of precursors involved in the production of these gases. Treated soil is placed both in the bottom of each incubation jar and in the eight (maximum) sample vials contained therein. The former serves as a continuous, undisturbed source of the measured gases while the latter provides convenient, preweighed samples for periodic chemical or microbiological analysis. Other features of the incubation system include (i) provision for anaerobic and sterile treatments, and (ii) automated air analyses and data acquisition controlled by a programmable timer that provides the flexibility to quickly and easily tailor the system's sampling, analytical, and recording functions to meet the specific needs of each experiment. Because the yield and composition of gases produced during soil N transformation processes is sensitive to incubation conditions, we propose that important features of this system be adopted by other laboratories to facilitate the comparison of research results by eliminating differences due to methodology.

Plants infected by mycorrhizae may exhibit increased root/fungal respiration and elevated concentrations of CO2 in the root zone. Carbon dioxide is a thermodynamically viable calcium phosphate weathering agent in calcareous soils. The weathering of phosphate minerals in soils releases P to the soil solution. This study was conducted to determine if differences in soil atmosphere CO2, of consequence to plant P availability, may exist between mycorrhizal and nonmycorrhizal plants. Western wheatgrass (Agropyron smithii Rydb.) was inoculated with raw soil that contained propagules of Glomus spp. and grown in prefumigated soil columns in the greenhouse. Soil atmosphere CO2 levels for inoculated plants averaged twice those of noninoculated plants in two separate experiments. The mean concentrations of CO2 in the soil atmosphere, measured at discrete weekly intervals, ranged between 0.13 and 2.63 kPa for inoculated systems and was significantly higher in 10 of 19 sampling events than for noninoculated systems (range of 0.05 to 1.02 kPa). Total plant P uptake was also significantly higher for inoculated compared with noninoculated plants. Total solution P, measured monthly in saturation throughput from experimental columns, was significantly higher in three of seven sampling events for inoculated compared with noninoculated plant systems. In treatments where noninoculated plants were grown under an imposed soil CO2 regime, total solution P was highly correlated with CO2. In soil systems where P availability is governed by the solubility of Ca-phosphate minerals, mycorrhizae may contribute to the P nutrition of host plants via a CO2-enhanced mineral weathering mechanism.

This work was supported in total by Grant BSR-8217358 from the Nat. Science Foundation.

In order to better characterize the behavior of denitrifying bacteria in soil and their effect on cycling of C and N, the rate of consumption and the extent of microbial assimilation of glucose were compared in aerobic and anaerobic denitrifying soils. Three soils (Bruno, a Udifluvent; Maury silt loam, and Paleudalf; and Lanton, a Haplaquoll) were amended with 14C-glucose and excess NO-3, then incubated for 3 to 7 d in air or anaerobically. Transient accumulations of soluble, nonglucose intermediates occurred in both aerobic and anaerobic soils. Aerobic substrate consumption rates were greater than anaerobic denitrifying substrate consumption rates. Anaerobic substrate consumption rate varied among soils more than aerobic rates, but the ratio of aerobic to anaerobic rates for a sample was not related to the presumed propensity of that soil type to denitrify. When all labelled glucose had been depleted, 8 to 10% more of the label had been mineralized to CO2 in the anaerobic than in aerobic incubations, suggesting lower efficiency of assimilation under denitrifying conditions. However, the effect of aerobic vs. denitrifying respiration on assimilation efficiency was small relative to differences among soil samples and the effects of substrate-to-soil ratio.

The investigation reported in this paper (no. 88-3-237) is in connection with a project of the KY Agric. Exp. Stn. and is published with the approval of the Director. This work was supported by a grant, BSR-8604964, from the NSF.

The effect of sludge amendment on the percentage of water-stable aggregates (WSA) of five soils with different physical and chemical properties was studied in an incubation experiment. A 5% sludge addition to soil incubated at 25 °C increased the per cent of WSA after 30 d in all of the soils. Bactericides were added to soil-sludge mixtures to assess the role of fungi in the aggregation process. Bactericide addition resulted in the suppression of bacterial growth and a concomitant proliferation of fungal hyphae. The WSA was highly correlated with water-soluble carbohydrates in all of the soils, and with hyphal length in two loessial soils. This suggests that cementing by fungal carbohydrates is a relatively more important binding mechanism than physical entanglement by mycelium in WSA formation of sludge-amended soils. By using a soil washing technique, it was determined that fungi involved in stabilizing aggregates were indigenous soil saprophytes which replaced the native sludge fungal flora.

This research was supported by grant no. WT 222 of the Israel National Council for Research and Development and the German Bundesministerium fur Forschung und Technologie. Contribution of the ARO, The Volcani Center, Bet Dagan, Israel. no. 2592-E, 1989 series.

Changes in microbial biomass dynamics and N cycling were studied in soils from a site derived from native grassland in western Nebraska managed under three tillage systems: no-till, stubble mulch, and moldboard plow (bare fallow). A treatment of native sod was maintained in each replication. The site was in native sod until 1970 when it was plowed and an alternate wheat (Triticum aestivum L.)-fallow rotation established. After 16 yr of cultivation, total N concentration in the top 10 cm of soil had decreased to 73, 68, and 50% of native sod in the no-till, stubble mulch, and plow treatments, respectively. Soil microbial biomass levels were decreased to 57, 52, and 36% for the corresponding tillage treatments as compared to the native sod. In laboratory incubation studies, CO2 respiration was proportional to microbial biomass. Nitrogen mineralization, however, was not proportional to microbial biomass. Immobilization of added 15N was highest for the native sod treatment and amounts of 15N extracted by KCl were inversely related to microbial biomass. Respiration rates per unit of mineralized N (r/Nmin ratio) increased in the order of plow, stubble mulch, no-till, and sod treatments, suggesting that C availability for microbial growth declined with increased tillage intensity. In these tillage experiments, increased tillage intensity decreased the ability of soil to immobilize and conserve mineral N.

Low-grade phosphate rock plus elemental S were added to an oxisol in combination with organic matter (OM) and/or Thiobacillus thiooxidans to study the change in P fractions with time and then study availability of P solubilized. The addition of any amendment caused a significant increase in Al-bound P. These values remained stable during the 7-wk incubation period. Soluble P and OM-bound P peaked within 3 wk and then decreased during the remaining period. The addition of T. thiooxidans plus S caused a rapid drop in soil pH to levels below 4.0 and a concurrent increase in P levels in the soil. After incubation, lime was added to the soils and sorghum grown for 30 d; even though, no additional P was incorporated. Solubilized P was sufficient for plant growth and dry matter produced by plants in soil amended with T. thiooxidans plus OM was similar to that of plants grown in soil amended with superphosphate.

Accurate prediction of yield response by corn (Zea Mays L.) to S fertilization on the coarse-textured soils of the Atlantic Coastal Plain requires information on availability and persistence of subsoil SO4-S, potentially mineralizable soil S, and S added in irrigation and precipitation. A 3-yr study was conducted with irrigated corn on four soils possessing characteristics commonly associated with S deficiency. A factorial combination of S application rate (0, 33, 67, 101 kg ha−1) and method (single broadcast at planting, split) was used. Grain yield, and S concentrations and N/S ratios of early whole plants (EWP) and ear leaves (EL) were determined. Extractable SO4-S (0–100 cm) levels and inputs of S from irrigation and precipitation were measured in each year; mineralizable S in Ap horizons of all soils was determined by two incubation methods (leached and nonleached). Although application of S generally increased plant S concentrations, significant yield increases occurred in only three of the 12 site-year combinations. Critical values for S and N/S, based on combined data from responsive sites, were 2.1 and 1.6 g S kg−1 and 18.7 and 20.3, for EWP and EL, respectively. Lack of yield response was attributed to subsoil SO4-S, (average, 0–100 cm = 170 kg S ha−1, mineralizable S (average = 80 kg ha−1, leached method) and S contained in irrigation or precipitation (annual average = 7.5 kg ha−1). Yield increases obtained at the most responsive site may have been caused by increased immobilization of S, due to no-tillage management, the presence of a physical barrier to root penetration at 40 to 60 cm, or subsoil Al. Successful prediction of corn response to S fertilization in Coastal Plain soils will require a comprehensive program that combines subsoil sampling and selective plant analysis, concentrated on sites identified as potentially responsive based on soil properties.

Soils of the Atlantic Coastal Plain commonly have thick sandy epipedons overlying loamy subsurface horizons. The origin of these sandy/loamy boundaries is often uncertain, due to frequent lack of visible indications of clay translocation. Five soils, which varied in depth to a sandy/loamy boundary, were analyzed for evidences of lithological discontinuity and clay translocation. Four of these soils occurred in close landscape association; the fifth soil, formed in an obviously stratified parent material, was included to test the sensitivity of the methods used. No evidence of a discontinuity was found for the four associated soils. Ratios of dominant sand subfractions to total sand were relatively uniform with depth, as was heavy mineral content of very fine sand. The stratified soil, however, exhibited considerable fluctuation in these parameters. Clay translocation was indicated by (i) the presence of oriented clay coatings on ped surfaces in the Btg horizon of one soil, as verified by micromorphological techniques; and (ii) significantly higher fine-clay to total-clay ratios below the sandy/loamy boundary than above. Results suggest that clay illuviation is a factor in the formation of sandy/loamy boundaries in some Coastal Plain soils even when the boundary is relatively deep (>50 cm).

This research was partially supported by state legislative appropriations (administered by Dep. of Agriculture and Consumer Services) and supplemental funds contributed by participating counties in support of the Florida Cooperative Soil Survey. Florida Agric. Exp. Stn. Journal no. 9253.

Research was conducted to evaluate Fe transformations in a hydrosequence of Oxisols from the Central Plateau of Brazil. The best-drained member of the hydrosequence of soils selected for the study classifies as an Anionic Acrustox (Dark-Red Latosol) and has codominant hematitic and goethitic iron oxide mineralogy and reddish hues (2.5 YR). X-ray diffraction and chemical extraction techniques were used to make detailed analyses of the iron oxides. Peak positions for hematite are coincident with those reported for hematite without foreign ion substitution. Peak positions for goethite are shifted toward a lower d-spacing, which corresponds to 34 mole % of AlOOH substituting for FeOOH. Soil samples from the Bo horizon were subjected to reducing conditions in the laboratory over a range of time periods, using sucrose as a source of energy to speed up the activity of the soil microorganisms. The observed changes in soil mineralogies confirm the preferential reduction and removal of hematite over goethite. Mineralogies and soil colors after reduction treatments are comparable to that of the Anionic Acrustox (Red-Yellow Latosol) found in the wetter section of the hydrosequence. The results of this study support the hypothesis that differences in color and iron oxide mineralogy of soils in this hydrosequence are the result of the preferential reduction of hematite over Al-substituted goethite in the presence of a seasonally high water table.

Contribution from the Dep. of Agronomy, Cornell Univ. Research supported in part by EMBRAPA-CPAC, the TropSoil/Cornell Univ. Program, and Hatch project 409, USDA.

Mass magnetic susceptibility (χ) was measured on samples taken from horizons from 27 northern California soils to determine the amount and vertical distribution of χ, and if the observed χ distribution could be related to parent material, climate, topography and time. In most of the soils with vertical morphological distinctions, eluvial horizon χ was greater than illuvial, R or CR χ. This susceptibility enhancement was interpreted as being due to pedogenic processes. Soils formed on parent material rich in Fe-bearing minerals (e.g. basalt) had higher absolute χ and greater χ enhancement than did soils formed on Fe-poor parent material (e.g. siltstone). Enhancement was greater in soils from areas of high (>1000 mm) mean annual precipitation compared with areas of low (<500 mm) mean annual precipitation, and soils formed under mean annual temperature ≤6 °C had less distinct enhancement than soils formed in warmer temperature regimes provided the precipitation was the same. Two soils with poor and somewhat-poor drainage classes had distinctly lower absolute χ than associated well-drained soils. Both absolute χ in eluvial horizons and susceptibility enhancement of eluvial over illuvial, and eluvial over C horizons increased as soils became older. Older soils were enhanced to a greater depth than younger soils. Eluvial and illuvial χ was greater than parent material χ in most profiles because non- or weakly magnetic minerals were converted to maghemite, which accumulated in eluvial horizons. In young soils with little morphological differentiation formed on transported parent material, χ measurements help to locate lithologic discontinuities.

Joint contribution from the Dep. of LAWR, Univ. of California, Davis, and the Dep. of Pedology, Inst. of Soils and Water, The Volcani Center, Bet Dagan. No. D2426-E, 1988 series, ARO.

Soils formed on alluvial fan deposits that range in age from about 35 000 to 200 yr BP near Silver Lake playa in the Mojave Desert permit study of the rates of soil development in an arid, hyperthermic climate. Field-described properties of soils were quantified and analyzed using a soil development index that combines properties and horizon thicknesses. We compared the variability in index values from five independent descriptions of the same soil profile with the variability in values from four soils of the same age described by different people. The variability due to description accounts for between 30 and 80% of the total variability (that due to both the soils and description) for the properties described, which emphasizes the need for consistent soil descriptions. Pedogenic CaCO3 (as indicated by color), pH increase, and dry consistence appear to change with age at linear rates, whereas rubification appears to change at a logarithmic rate. The linear rates are best attributed to the progressive accumulation of CaCO3- and salt-rich eolian dust derived from the playa and other more distant sources. The total-texture values of soils on fans older than 10 000 yr BP are similar, which suggests that playas in this area may have been wet enough to restrict the availability of fines from these sources for many thousands of years prior to 10 000 yr BP. Index values for most properties of soils on the lower fans adjacent to the playa increase at faster rates than those of soils of the same age on the upper fans farther from the playa, probably due to the proximity to this local dust source. Equations derived from regressions of soil age and properties can be used to estimate ages of undated, lithologically similar deposits in similar climates and geomorphic settings.

An Aquic Fragiudalf developed from a fluvial terrace of Pleistocene age in northwestern Italy was investigated to determine the possible role of different compounds in fragipan bonding. Fragipan characteristics appeared only in yellow, goethite-containing zones of the fragipan horizon but not in gray, essentially Fe oxides-free zones. Citrate-bicarbonate-dithionite (CBD), but not acid oxalate, extracted more Si in the yellow zones than in the gray zones. These data, and the fact that citrate-bicarbonate (CB) without dithionite extracted very little Si demonstrate that CBD extracted a Si compound specifically associated with the Fe oxides (mostly goethite). In the fragipan the ratio CBD-extractable Si goethite reached a profile maximum suggesting that the Si compound associated with the goethite particles could, if present in sufficient amount, cement goethite particles together thus providing a framework to mechanically hold other soil particles; alternatively, that compound could bind goethite with other soil mineral particles. Although the amounts of CBD-extractable Si were low, a simple calculation showed that they were clearly in excess of those needed to saturate the theoretical sorption capacity of goethite, giving a surplus of Si able to promote bonding. Selective extraction methods provided no evidence for implication of Al in bonding. Consequently amorphous silica could be the bonding agent.

The North Carolina Piedmont has been farmed for nearly 200 yr and there is considerable concern about the effect of soil erosion on the productivity of Piedmont soils. Most attempts to assess the effect of erosion on soil properties and productivity have few comparable virgin areas to use for baseline data. Fortunately, a 12-ha virgin tract, mapped as Cecil sandy loam, B and C slopes, was located on the Guilford College campus in Greensboro, NC. The purpose of our study was to quantify the physical and chemical properties of a virgin Cecil soil map unit and to compare soil properties and distribution to that of cultivated sites. Detailed studies of the surficial sediments, geomorphology and soil morphology were made along five transects run at right angles to the topographic trend. Maps of hillslope sediment thickness and geomorphic surfaces were made using the information from the transects and bore holes. Pedons were sampled by landscape position for laboratory analyses. Horizon thickness and depth to residual material varied with landscape position. Nutrient status of all virgin soil horizons are very low, especially when compared to cultivated sites.

Contribution from the Dep. of Soil Science, North Carolina State Univ. Paper no. 11647 of the Journal Series of the North Carolina Agric. Res. Serv., Raleigh, NC 27695-7643. Student labor and supplies supported in part by a grant from the USDA-SCS to the Dep. of Plant Science, NC A & T State Univ.

The Direct/Delayed Response Project (DDRP) is estimating the number of lakes and streams in three U.S. regions that might become acidic due to current or altered levels of acidic deposition, and the long-term time scales involved. Because of the influence of soils on aquatic chemistry, DDRP acquired data on soils that were mapped, sampled, and analyzed consistent methods across the regions. In the northeastern USA, about 600 soils (mainly phases of soil series) were identified during mapping of 145 watersheds. Because statistically adequate sampling of every soil was impractical, the soils were grouped into 38 sampling classes. Each of these classes was sampled across several (usually eight) watersheds. The properties of soils on specific watersheds (or portions of watersheds) can be estimated from the regional means and variances of the sampling classes and the percent occurrence of sampling classes on each watershed. This paper describes how the sampling classes for the northeastern USA were developed, the definitions of the classes, and the characteristics of soils within the classes. A preliminary statistical test indicates that the occurrence of sampling classes on watersheds is a significant predictor of Acid Neutralizing Capacity class of the corresponding lakes. Final evaluation of the utility of this scheme will come from the very extensive data analysis and modeling tasks of the DDRP.

Semivariograms are used to quantitatively assess spatial variability of depth to mottles, depth to gravels, and thickness of loamy sand and/or coarser-textured layers, which are definitive criteria for classification of soils derived from alluvium on the Canterbury Plains near Lincoln College, New Zealand. The three properties vary anisotropically with the anisotropy ratio being highest for depth to mottles (k = 5.84), lowest for thickness of loamy sand and/or coarser-textured layers (k = 1.58), and intermediate for depth to gravels (k = 2.43). Directions of maximum variation for depth to mottles and depth to gravels are NE-SW across an abandoned channel hollow. This pattern is reflected in the soil map of the study area and in the smaller-scale soil map of the adjacent region. Such variation reflects the past regional drainage patterns of channels flowing predominantly in a NW-SE direction. The appropriate field configuration of a sampling scheme for future survey of similar adjacent soils would be rectangular with a sample spacing in the direction of least variation k (anisotropy ratio) times that in the direction of maximum variation. Suitable sample numbers and sampling intervals to achieve desired levels of precision in the direction of maximum variation are determined. These are obtained from graphs showing relationships between kriging standard error, sample spacing and sample number. This geostatistical approach is more efficient than conventional statistical methods in designing sampling strategies: less samples are needed for kriging than for the conventional method to achieve the same level of precision.

Contribution of the Dep. of Soil Science, Lincoln College, New Zealand and N.Z. Soil Bureau, DSIR, Christchurch, New Zealand.

Forest soils in the loess-mantled ridge and ravine topography of the Driftless Area of southwest Wisconsin show minimal effects related to hillslope gradient. Twelve undisturbed Fayette silt loam profiles (Typic Hapludalf, fine silty, mixed, mesic) that developed in Peoria loess and silty colluvium on slopes of 2 to 55% were examined to ascertain systematic differences as a function of slope. Climate, vegetation, parent material, and time of development were similar for the 12 profiles. Physical and chemical properties of the A and E horizons (including texture, pH, dithionite extractable Fe and Mn, and total P) do not vary significantly as a function of slope gradient. Of the B and C horizon properties measured, only Bt horizon pH, clay accumulation, and total P distribution show statistically significant variations with slope gradient. The Bt horizon pH decreases with increasing slope gradient (r2 = 0.63). Illuvial horizons show less clay accumulation in profiles on steeper slopes (r2 = 0.37). Total P content is higher throughout the B and C horizons of profiles on steep slopes (45%) compared to soils on level and moderate slopes (2–20%). Together these results indicate slightly less leaching of the subsoil on steep slopes. Exchangeable Ca/Mg, Fed, Mnd, maximum clay, and depth to maximum clay reveal no trends as a function of slope gradient. Soil conditions have favored infiltration and minimized surface erosion, leading to nearly uniform soil development in homogeneous parent materials on ridgecrests and sideslopes.

Low water infiltration caused by crust formation during rain or sprinkler irrigation is a significant problem in some arid and semiarid regions. Polymers may be applied in irrigation water through a sprinkler system, but must be applied directly to the soil under rainfall conditions. The objectives of this rainfall simulator study were to: (i) determine the effect of drying of crusted vermiculitic soil on the subsequent crust properties and infiltration rate (IR) values, (ii) determine the effect of polymers applied at low concentration in irrigation water of two qualities on the IR under consecutive water applications, and (iii) determine the effectiveness of polymer application to the soil as would be required under rainfall conditions. Two cationic polysaccharide guar derivatives having a higher (HCCP) and a lower (LCCP) charge density and a polyacrylamide (PAM) with a low negative charge density were used in the study. Applications of polymers with the sprinkler water maintain IR in the order HCCP > LCCP > PAM > untreated. Except for PAM, the polymer applications were relatively ineffective in subsequent sprinkler applications with plain water applied with impact energy. The beneficial effects were preserved under water application without impact energy. Spraying concentrated polymer solutions on the soil surface was not effective in preventing crust formation by following rain events except for the case when LCCP was sprayed on in a CaCl2 solution. The results are explained on the basis of polymer adsorption and penetration into the soil surface layer and aggregates.

Spatial soil heterogeneity poses problems on field experiments which use large treatment plots, as in tillage research. Recent studies have established that many soil properties do not vary in a random manner, but often exhibit spatial dependence. Presently available methods to account for spatial effects either do not fully address these problems or they are too complicated to be widely accepted. The purpose of this study was to develop an experimental design which was based on the principles of regionalized variable theory. Using incomplete blocks of size two, the method estimates treatment effects by means of short-distance comparisons; this prevents inflation of the error term due to spatial soil variability and eliminates biases caused by variable distances between plots. The analysis of variance for the method is simple and well established. A Monte Carlo simulation study using uniformity trial data was performed to compare the proposed method with a randomized complete block design. The study revealed that the use of incomplete blocks reduces the average error mean square by 44%, and the average coefficient of variation by 25% compared to using complete blocks. The proposed method was, on the average, 21% more efficient then a complete block design.

Contribution of the N.C. Agric. Res. Serv. Paper no. 11431 of the Journal Series of the North Carolina Agric. Res. Serv.

Infiltration characteristics of a Port Byron (fine-silty, mixed, mesic, Typic Hapludoll) silt loam soil located in the karst terrain of southeastern Minnesota were determined using both ponded water and simulated rainfall. Three tillage treatments, with and without surface cover, were studied to provide a range of soil physical conditions. Simulated rainfall was applied after various amounts of both natural and artificial rain had fallen since tillage. Large differences in infiltration characteristics were attributed to the development of a surface crust. In the absence of a crust, this soil was highly permeable (>200 mm h−1) while surface-crusted infiltration rates were as low as 10 mm h−1. Infiltration characteristics were related to depth of rainfall since tillage, cover, and random roughness. Statistical analysis showed that >77% of variation in infiltration rate, Green and Ampt hydraulic conductivity, and curve number was explained by the depth of rainfall since tillage, surface cover, and random roughness. A procedure is suggested that describes the change in infiltration characteristics as a function of rainfall since tillage for various tillage conditions.

Claypan soils with poor physical properties are the only surface materials available to cover and reclaim much of the spoil left by strip mining for coal in east-central Texas. We compared an undisturbed claypan soil profile of Axtell fsl fine montmorillonitic, thermic Udertic Paleustalfs) to four constructed soil profiles and evaluated them as cover soils for toxic minespoil. We compared profiles constructed from minespoil selected for low pyrite content alone (non-topsoiled) against the same selected spoil covered by a mixture of Axtell soil material. The rainfall intake rate and physical properties of the materials controlled forage production by kleingrass (Panicum coloratum L.). All of the constructed soils stored less water in the soil profile than the undisturbed claypan soil, and forage yield was reduced on them during drought. Infiltration rate during the first 2 cm of rainfall influenced forage yield more than final intake rate. Mulch application increased water storage and forage yield up to 15% on selected minespoil, but not on the mixed claypan soil profiles. Either selected minespoil or a mixture of Axtell soil may be used to cover toxic random soil with a nontoxic layer, but the Axtell mix had the least acid-forming potential. Selected minespoil material was equal in value to a mixture of the 1.8 m claypan soil profile as a surface material for constructed minesoils.

A compacted layer at 0.15 to 0.25 m frequently occurs in soils of the southern Piedmont. This hardpan can be caused by disk or other tillage but the effect that wheel traffic may have in the formation of this layer has not been shown. The objectives of this study were to determine the extent traffic may contribute to hardpan formation, the effectiveness of shallow in-row chiseling, and the possible correlation of the hardpan with a morphologically identifiable soil horizon. A long-term tillage experiment where traffic had been controlled was examined at the end of its eighth year. The soil was a slightly eroded Cecil sandy loam (clayey, kaolinitic, thermic Typic Kanhapludult). Summer crops of soybean [Glycine max (L.) Merr. ‘Ransom’] and grain sorghum [Sorghum bicolor (L.) Moench, DeKalb BR-64] were double-cropped with winter wheat (Triticum aestivum L. Thell, ‘Coker 747’, 1979–1982, ‘Stacy’, 1983–1986). Spring tillage treatments consisted of disk tillage (DT), in-row chiseling (IC), and coulter planting or reduced tillage (RT). All tillage treatments were disked in the fall before planting wheat. Cone index was measured in 1987 and 1988 in five positions relative to the row and showed that a hardpan was present in all positions at 0.15 to 0.25 m. The hardpan was a result of disk tillage and wheel traffic. Traffic caused compaction to a depth of 0.28 and 0.18 m in DT and RT, respectively. In-row chiseling was effective in disrupting the hardpan to a depth of 0.26 m and contributed to significantly increased yields in grain sorghum. The hardpan coincided with the Ap2 horizon in this soil. The top of the Bt horizon may act to confine compactive stresses to the soil horizon above.

The effect of slope on runoff and erosion has been studied mainly in stable soils. Loamy soils from semiarid regions have unstable structures and tend to seal during a rainstorm. The permeability of the seal is sensitive to water quality. The effect of slope angle (5–30%) and addition of phosphogypsum (PG), which changes the water quality, on the infiltration rate (IR), runoff and erosion from an unstable sandy loam soil material (Typic Rhodoxeralf) was studied using a rain simulator. Increasing the slope slightly reduced the amount of runoff and increased the final infiltration rate. The increase in final IR was due to seal erosion. The PG application increased the permeability of the seal, tripled the final IR of the soil sample and decreased the volume of runoff by 50%. Phosphogypsum application also reduced erosion by 60% at the gentlest slope angle. Change in slope angle from 5 to 25% doubled soil loss in the PG-treated soil samples but increased by seven-fold soil loss from the untreated soil samples. Gypsum treatment releases electrolytes into the percolating and runoff water. Its effect on soil erosion is due to decreasing the fraction of runoff water, stabilizing the soil structure at the soil surface, and increasing the rate of sediment deposition. The dramatic effect of PG in reducing erosion from steep slopes may be used in stabilizing soil structures on steep slopes.

Conservation tillage systems utilizing winter annual cover crops represent a different soil physical environment compared to conventional tillage systems. A field experiment was conducted for 3 yr on a Goldsboro fine sandy loam (fine-loamy, siliceous, thermic Aquic Paleudults) in the North Carolina Coastal Plain to assess effects of cover crop type and row position on soil physical properties under no-tillage corn (Zea mays L.) management. Bulk density (ρb), soil porosity, and hydraulic conductivity (Ksat) were measured in fallow, winter wheat (Triticum aestivum L.), and hairy vetch (Vicia villosa Roth.) systems with respect to three row positions (trafficked, untrafficked, and plant row). All traffic was controlled such that each corn row was bordered by a trafficked and untrafficked interrow. In general, soil physical properties were unaffected by cover crop type but strongly influenced by position. Bulk density was significantly higher in the trafficked vs. untrafficked position (1.74 vs. 1.52 Mg m−3) after 3 yr and tended to increase with time in the trafficked interrow. Associated with higher ρb values in the trafficked interrow were significantly lower values for soil porosity and Ksat. Total porosity in the trafficked position, averaged over cover crop type and 3 yr, decreased 21% below that of the untrafficked position. After 3 yr, Ksat was 0.019 and 0.002 mm s−1 in untrafficked and trafficked interrows, respectively. These results suggest that controlled traffic patterns may be an important component in the management of continuous, conservation tillage systems.

Soils are more vulnerable to erosion following cropping to soybean (Glycine max [L.] Merr.) than corn (Zea mays L.). This has been attributed to lower dry matter production, less residue cover, and soil-loosening action by soybean roots. To augment soil cover, common chickweed (Stellaria media L.), Canada bluegrass (Poa compressa L.), and downy brome (Bromus tectorum L.) were grown as winter cover crops with no-till soybean on natural rainfall erosion plots located on a poorly drained Mexico claypan soil (Udollic Ochraqualf). No-till soybean without a cover crop served as the check. Winter cover crops significantly increased soil cover by 30 to 50% during the critical erosion period of late spring to early summer. Compared to the check, mean annual soil losses from the chickweed, downy brome, and Canada bluegrass were decreased by 87, 95, and 96%, and runoff was reduced 44, 53, and 45%, respectively. Dissolved NH+4 concentration in runoff from cover crops was 1.61 to 3.72 times more, and dissolved PO3-4 was 1.61 to 2.86 times more than that of the check. However, runoff from the check plots had 96 to 117% greater concentration of dissolved NO-3 than cover crop plots. Mean annual dissolved nutrient losses were decreased 7 to 77% by using winter cover crops. Thus, winter cover crops were very effective in reducing soil erosion and dissolved nutrient losses from no-till soybean.

Contribution from the MO Agric. Exp. Stn. Journal Series no. 10682. This work was conducted under Missouri Agric. Exp. Stn. Project 396, with the financial support in part by the Dep. of Interior, U.S. Geological Survey, through Missouri Water Resources Research Center (USDI-GS-14-08-0001-G1235), by USDA-ARS (58-6125-5-15), and by the Missouri Dep. of Natural Resources (MDNR-86-4-0).

Research on soil surface sealing and crust formation has taken two approaches: (i) measurement of common soil erosion parameters such as infiltration, permeability, detachment, and strength and (ii) observation of seal or crust morphology using both microscopic and submicroscopic techniques. The purpose of this research was to combine the two approaches to extend our knowledge of surface sealing and crusting and its role in the erosion process. Eight soils with exchangeable sodium percentages (ESP) <1 and with textures ranging from sandy loam to clay were exposed in the laboratory to simulated rainfall with an intensity of 63 mm h−1 for 1 h. Seal and crust formation were investigated by measuring splash and wash erosion, single waterdrop splash, and fall-cone strength and by observing scanning electron microscope (SEM) and thin-section photographs. Medium-textured soils (five silt loams and a silty clay loam) developed homogeneous, silty surface seals of high strength ranging from 14 to 43 kPa and low splash detachment. Splash amounts for single drop impact ranged from 0.8 to 1.8 mg soil per drop, and for multiple raindrops in erosion pans, from 9 to 26 g soil during the final 5 min of a 60 min rain. For a clay soil, no morphological evidence of crusting was observed; strength of the seal was low (6.4 kPa) and splash amount was high (40.0 g in 5 min). For a sandy loam soil, splash was high (41.8 g) and strength, low (10.8 kPa). Crust morphology indicated slight crusting. The subjective nature of interpreting SEM and thin section photographs makes it difficult to draw conclusions about erosion processes or rates.

Moisture and nutrient conditions are poorly characterized in soils at elevations >1500 m in the southern Appalachian Mountains. In the Black Mountains, high elevation soils are Typic and Lithic Haplumbrepts, with umbric epipedons that are extremely acid, organic-rich, rocky, and unstable due to the steep slopes. Many of the Umbrepts in the Black Mountains have been disturbed by exploitative logging, repeated wildfires, and depredation by the balsam woolly adelgid (Adelges piceae Ratzeburg), each of which has caused major fluctuations in C, nutrient, and hydrologic cycles of soils and ecosystems. The objective of this study was to evaluate predictions based on climate, forest disturbance, and soil genesis: that these soils are rarely subjected to low water potential, that soil N mineralization rates are currently high, and that availability of soil nutrient cations is low. A water balance model appropriate for soils with average water-holding characteristics indicated that, on a 6-yr recurrence interval, plants deplete soil moisture to <-0.2 MPa during low rainfall periods of one-month duration. High rock contents (about 0.40 m3/m3 of soil volume in 40-cm depth) limit soil water storage capacity, and make the spruce-fir forests very dependent on regular rainfall supplies. Soil N appears mineralizable at moderately high rates, as indicated by three soil and plant indices of N availability, whereas plant availability of Ca and Mg appears marginal. Exchangeable Ca and Mg total only 6.4 and 3.4 kmolc/ha, respectively, in the surface 40 cm of mineral soil, low contents that indicate rapid rates of biogeochemical cycling of divalent cations in these ecosystems. The long-term recovery of these soils from 20th century disturbances depends directly on the dynamics of soil organic matter, due to organic matter's susceptibility to disturbance and to its control over soil moisture and nutrient availability.

Plant and soil processes involving N-transformations have been monitored by techniques measuring the 15N/14N ratio in a sample relative to the atmosphere (δ15N). Unusually low δ15N values have been reported in the tissues of chaparral shrubs. The primary objective of this study was to investigate soil N-cycling processes which may yield the low plant tissue δ15N levels. Since the chaparral is subject to periodic brush fires, which in turn result in high levels of inorganic N-forms in the soil during the first year following the burn, possible effects of burning on soil δ15N were also investigated. Incubations were conducted on soils from an area which had been subjected to a brush fire, and an adjacent unburned area; both soils are fine, thermic, schistose, very steep Ultic Haploxeralfs. Concentrations and δ15N of soil NH+4 and NO-3 were periodically measured. A C- and N-rich ash resulted in rapid mineralization of N in the burned soil; a substrate more resistant to biological degradation resulted in an initial loss, then subsequent slow accumulation of inorganic-N in the unburned soil. Nitrate was the dominant mineral species in each soil after a few weeks. As nitrification progressed, δ15N of NH+4 increased and δ15N of NO-3 decreased. Since the mineral pool in each soil became dominated by NO-3, δ15N of the mineral pool became strongly negative. A mathematical model of isotope dynamics fits empirical data well.

This work was supported by the State of California Air Resources Board.

The relationships between soil water potential and water table depth in the upper five horizons of an Ultic Haplaquod of the lower Coastal Plain flatwoods are described. Soil water pressure head in the E, Bh, Bs, and E′ horizons was in hydrostatic equilibrium with the underlying water table during all conditions when water was perched above an argillic horizon. The A horizon was in equilibrium with the water tables only when water tables were <65 to 70 cm from the soil surface. Water pressure heads and contents in the soil profile were simulated using a finite-difference, one-dimensional, unsaturated water-flow model. Simulations of water pressure head were excellent for the lower horizons (E through E′) under all conditions, and poor for the A horizon. During high water tables, water pressure head simulations for the A horizon were improved. Fewer discrepancies between measured and simulated conditions occur when water pressure head values are converted to volumetric water contents. Unfortunately, the water-flow model described here did not adequately handle high infiltration rate rainfall events experienced at the site. It is concluded that in these sandy soils: (i) water content is the preferred prediction variable; and (ii) further advances in mechanistically modeling water uptake from each soil horizon in forest ecosystems will be limited by adequate information on plant root biology.

A contribution of the Soil Science Dep., Univ. of Florida, Journal Series no. 9386.

Processes involving the movement of organic substances in forest soils are not well understood. This study was conducted to examine the role of acidic inputs on dissolved organic carbon (DOC) mobility, processes affecting the retention of DOC by a B horizon, and SO2-4 adsorption. Using O and B horizon samples from a Spodosol collected in a forested watershed in Maine, acid solutions leached through O, B, and O over B (O/B) soils in small vacuum extractor columns were analyzed for DOC, DOC fractions, and anions. For the O and O/B columns, DOC in the leachates decreased (7550 to 3350 and 2380 to 850 µmol C L−1, respectively) with an increase in acidic inputs; for the B horizon, the reverse was the case (435 to 1570 µmol C L−1). The DOC in leachates from the O horizon was dominated by hydrophobic and hydrophilic acids (68 and 20% of DOC with no acid addition), which were altered by acidic inputs (46 and 36% of DOC at p[H+] = 2). The hydrophobic acid percentage in leachates from O/B horizon columns decreased from 52 to 28%, whereas hydrophilic acids increased from 27 to 47% with H2O and p[H+] = 2 treatments, respectively. Ionic strength adjustment (I = 0.01) of treatment solutions reduced C solubilization in O and O/B horizon leachates, leading to lower DOC leaching. The SO2-4 adsorption in O over variable B horizon columns decreased with greater B horizon mass due to pH dependent SO2-4 adsorption processes. The B horizon adsorption of DOC from field collector leachates showed that hydrophobic acids were selectively adsorbed, as indicated by initial mass isotherm coefficients. The results indicated that acidity, salt effects, and B horizon mass are significant factors in controlling both the amount and composition of DOC in surface and groundwaters.

Because of their remote location and relatively limited extent, soils in the southeastern spruce-fir zone have not been extensively characterized either physically or chemically. The objective of this study was to describe and characterize the soils found in most of the major spruce-fir areas in the Southeast as a first step in evaluating potential growth changes in the region. Fifty-two soil profiles, randomly distributed within the spruce-fir cover type, were described and samples were collected for physical and chemical analysis. Soils sampled were classified into the following taxonomic subgroups: Lithic Borofolists, Umbric Dystrochrepts, Lithic Haplumbrepts, Typic Haplumbrepts, Pachic Haplumbrepts, and Lithic Haplorthods. The data set was further condensed by focusing only on the rooting zone as defined for each pit by field measurements. Mean rooting depths ranged from 17 to 39 cm. All profiles were strongly to extremely acid (pH 3.6–4.6), with CECs in the 28 to 39 cmol kg−1 range and base saturation less than 4%. Considerable variability was seen among soils in organic matter (132–312 g kg−1) and total Kjeldahl N (TKN) content (5.4–11.0 g kg−1). Statistically significant (P = 0.05) differences also occurred in exchangeable base cations, while total cations exhibited less variability among groups. Extractable P varied little among these subgroups with only the Pachic Haplumbrepts having significantly higher concentrations. Total P exhibited more variability in concentration. Acetate-soluble SO4-S ranged in concentration from 0.08 to 0.67 g kg−1. Extractable and exchangeable Al concentrations were generally lowest in the Lithic Borofolists and highest in the Lithic Haplumbrepts. Observed variations are attributed primarily to differences in organic matter accumulation and to the impact of soil creep/soil mixing due to general slope instability.

Subsurface soil acidity resulting from repeated NH3 applications in long-term conservation tillage systems that do not disturb the NH3 injection zone was studied in six different crop-fertilizer-tillage systems by: (i) observation of the size, shape, and distribution of acidic zones in the field by using a pH color indicator method and (ii) intensive quantitative sampling of the upper 25- to 30-cm soil layer with subsequent laboratory analysis for soil pH. Acidic soil zones created by the nitrification of the injected NH3 were roughly circular with 12- to 18-cm diam. Soil pH of the acidic zones was generally 0.9 to 1.8 pH units lower than that of the surrounding bulk soil. In the ridge till-plant and ridge slot-plant systems studied, a distinct highly localized persistent acidic soil zone was detected in each interrow. On the basis of the extent and degree of acidity observed, it is concluded that yield-limiting problems due to acidification by continuous NH3 applications are not likely in the ridge management systems studied. In the flat no-till systems studied, numerous persistent acidic soil zones were observed scattered throughout each interrow. It is concluded that soil acidity problems due to long-term NH3 usage potentially could develop in the no-till systems studied where NH3 is not injected in the same vicinity each year. Efforts toward localized placement of N by the farm operator could effectively minimize potential problems due to the acidifying effects of NH3 in conservation tillage systems.

Journal Paper no. J-13018 of the Iowa Agric. & Home Econ. Exp. Stn. Project 2555. Supported in part by a grant from Dow Chemical Co.

The movement of N in relation to water movement and plant uptake is, in part, determined by the rate of nitrification. Nitrifying bacteria are known to be inhibited by low pH. A laboratory incubation and a field experiment were conducted to study nitrification of NH4 supplied by ammonium sulfate (AS), diammonium phosphate (DAP), and urea, N sources that produce environments with different pH, particularly when applied to moderately acidic, poorly pH-buffered soil. The field site was located on a texture contrast soil (sandy, thermic Natric Durixeralf) at Merredin, Western Australia. The incubations were performed on a sample of the surface 100 mm of this soil. The initial pH and the pH-buffer capacity of the sample were 5.4 and 4 mmol kg−1 (pH unit)−1, respectively. Nitrification rates were found to vary with the inherent alkalinity of the N source, viz., urea > DAP > AS. The nitrification process was modeled taking into account the effect of pH on nitrifier growth and activity. A linear relation in the model between pH, and both the specific growth rate and activity of the nitrifiers accounted for most of the variability between N sources. In the field experiment, 15N remained largely in nonnitrate form for the first month after application, presumably because of low temperatures, but after 53 d production of NO3 was greater with urea than AS. With the slow rate of nitrification, the leaching of NO3 was minimal.

This research was partially funded by CSBP and Farmers, and the State Wheat Res. Committee of Western Australia.

Field studies were conducted on a Crowley silt loam (Typic Albaqualf) to investigate the influence of split applications, subsurface placement, and the nitrification inhibitor (DCD) dicyandiamide (cyanoguanidine) on the plant uptake of 15N-labeled urea by dry-seeded rice (Oryza sativa L. ‘Newbonnet’). Fertilizer N topdressed in three split applications (TD3SA) resulted in the highest rice grain yields and highest percent recovery of fertilizer N in the rice plant. Fertilizer N applied all preplant with DCD resulted in slightly less grain yield, but similar total dry matter yield, total N uptake and fertilizer N recovery in the soil-plant system as the TD3SA method. Fertilizer N applied all preplant without DCD was inferior to the other treatments. Subsurface preplant incorporation (SSPPI) of N resulted in greater fertilizer N uptake by the rice plant compared to broadcast preplant incorporation (BCPPI); but, this did not translate into more total N utilized or grain produced. It appeared that DCD, by maintaining more of the fertilizer N as NH4-N, caused less fertilizer N to be lost via denitrification after flooding and may have enabled more of the fertilizer N to participate in the mineralization-immobilization turnover cycle and exchange for soil N. Overall, DCD increased the uptake and yield effectiveness of preplant N in dry-seeded rice cultivation.

Published with permission of the Director of the Arkansas Agric. Exp. Stn. Project ARK-01047. Supported in part by the Tennessee Valley Authority and the Arkansas Rice Research Board.

Management practices, designed to reduce ammonia (NH3) volatilization and total N loss from flooded rice fields, after application of urea, were assessed in the dry season at Mabitac, Philippines. The assessment was made in replicated 4.8-m × 5.2-m plots by determining NH3 and total fertilizer N losses with bulk aerodynamic and 15N balance methods, respectively. The bulk aerodynamic method was first evaluated by comparison of ammoniacal-N concentrations, pH values, and equilibrium NH3 concentrations for a 50-m diam, circular area and small plots receiving the same fertilizer treatment. Good agreement was obtained so the bulk aerodynamic method was used to determine NH3 loss from the small plots receiving the different management treatments. Ammonia loss was high when urea was broadcast into floodwater 10 d after transplanting—48 and 56% of the applied N was volatilized as NH3 during the first 8 d after application of 53 and 80 kg N ha−1, respectively. Total N losses were 60 and 59%, respectively. Ammonia loss was reduced slightly (to a mean of 43% of the applied N for the two application rates) by broadcasting the fertilizer into 0.05-m-deep floodwater and incorporating it into the soil by harrowing before transplanting. Removal of the floodwater before applying urea onto saturated soil and incorporation by harrowing reduced NH3 loss to a mean of 9% and total fertilizer N loss to a mean of 33%. Fertilizer N recovery by rice plants was significantly increased by incorporation without surface water.

The objective of this study was to predict the most profitable amount of P fertilizer applied to winter wheat in terms of grain yield increase as influenced by soil test P, method of P application, and projected yield level. Winter wheat (Triticum aestivum) was grown in 22 field experiments between 1978 and 1982 with treatments consisting of six P rates applied with the seed and broadcast. The soils were Keith-Holdrege and Uly-Colby silt loam (fine-silty, mixed mesic Typic Argiustolls). Yield increase to applied P was related by multiple regression to rate and method of P application, soil P level, soil pH, and grain yield level (R2 = 0.76, SE = 0.20 Mg ha−1). This function estimates maximum profit at specific fertilizer and grain prices. The regression function provides a mathematical solution for incorporating yield goal into fertilizer P recommendations. The model shows that P requirements for winter wheat increase as the yield level increases. Most importantly, the model indicates that it is economically prohibitive to follow the traditional recommendations of doubling the rate of broadcast P compared to seed applied P for winter wheat. When grain yields are increased with applied P, seed applied P results in much higher grain yield increase per kilogram of applied P than broadcast P. This results in being able to pay for more P when seed applied than broadcast. At medium and low levels of soil P, seed application resulted in 2 to 4 times more profit from fertilizer P application than broadcast P.

Contribution of Nebraska Exp. Stn. Journal no. 000. This research was supported in part by Tennessee Valley Authority.

The surface charge properties of x-ray noncrystalline aluminosilicates in relation to their structural distortion by citric acid during their formation were investigated. The precipitates formed from solutions containing monomeric orthosilicic acid and AlCl3 (Si/Al molar ratio 0.5 and OH/Al molar ratio 2.0) in the presence of citric acid (citrate/Al molar ratios of 0.02 and 0.025) exhibit an IR absorption maximum at 965 cm−1, a feature similar to that of “allophane-like constituents” or “ill-defined fraction of the allophane-imogolite complex” in soil. The complexation of citrate ligand with hydroxy-aluminosilicate ions or “proto-imogolite”, the precursor of paracrystalline to x-ray noncrystalline aluminosilicates, promoted the development of structural defects apparently through perturbation of the regular linkage of hydroxy-aluminosilicate fragments. The perturbation effect of citrate ligands promoted the development of positive charge and resulted in a marked increase in the point of zero charge (PZC) due to the exposure of Al-OH0.5+2 groups around the defect sites. The role of complexing organic ligands in influencing the surface properties of short-range ordered aluminosilicates through structural distortion thus deserves attention.

Contribution no. R600, from Saskatchewan Institute of Pedology, Univ. of Saskatchewan. Research supported by the Natural Sciences and Engineering Research Council of Canada, Grant A2383-Huang.

A procedure is described which allows the ionic composition of the soil solution and exchange phases to be measured on the same small soil sample. This procedure is based on existing methods which use a dense, water-immiscible liquid to displace soil solution during high speed centrifugation. Exchangeable ions are then extracted by shaking the soil with 1.0 mol L−1 ammonium acetate (pH 7), after which the soil is oven-dried and the water content determined. The solution extracted from the soil sample accurately represents the ionic composition of the equilibrium soil solution and the procedure does not affect the ionic composition of the exchange phase.

Ion exchange equilibria involving K cannot be described by a constant selectivity coefficient for all surface compositions. The main reason is that the soil exchanger contains various types of sites with different affinities for K ions. This paper makes use of a multi-site model, the main assumption of which is that the exchange reaction is governed by single values of the Vanselow selectivity coefficient, Kv, for each type of site. The simplest model—i.e., the two-site model—is tested. Three parameters must be adjusted: the proportion of high and low selective sites, and the Kv's for these two classes of sites. Simulated K-Ca exchange isotherms are shown, with various combinations of the parameters. The model has been adjusted to experimental K-Ca exchange data, using a non-linear regression procedure. A fair agreement between the experimental and calculated curves is obtained. As a consequence, this simple model proves to be a powerful tool for a quantitative description of exchange data involving K ions.

A probe-type ultrasonic vibrator was used to disperse, without chemical pretreatments, samples of three soils developed from volcanic ash. The efficiency of dispersion by ultrasonic vibration was evaluated using curves of percent clay or sand vs. vibration time along with electron micrographs of sand (50–2000 µm), coarse silt (20–50 µm), and clay (<2 µm) size fractions. Ultrasonic vibration for 900 s at an energy of 39.9 W/cm2 was sufficient to disaggregate the soils without significantly damaging primary sand- and silt-sized glass shards or destroying imogolite threads in the clay fraction.

Contribution of the College of Agriculture and Home Economics Res. Ctr. Scientific Paper no. 7677.

A probe-type ultrasonic vibrator was used to disperse, without chemical pretreatments, samples of three soils developed from volcanic ash. The efficiency of dispersion by ultrasonic vibration was evaluated using curves of percent clay or sand vs. vibration time along with electron micrographs of sand (50–2000 µm), coarse silt (20–50 µm), and clay (<2 µm) size fractions. Ultrasonic vibration for 900 s at an energy of 39.9 W/cm2 was sufficient to disaggregate the soils without significantly damaging primary sand- and silt-sized glass shards or destroying imogolite threads in the clay fraction.

Contribution of the College of Agriculture and Home Economics Res. Ctr. Scientific Paper no. 7677.

A probe-type ultrasonic vibrator was used to disperse, without chemical pretreatments, samples of three soils developed from volcanic ash. The efficiency of dispersion by ultrasonic vibration was evaluated using curves of percent clay or sand vs. vibration time along with electron micrographs of sand (50–2000 µm), coarse silt (20–50 µm), and clay (<2 µm) size fractions. Ultrasonic vibration for 900 s at an energy of 39.9 W/cm2 was sufficient to disaggregate the soils without significantly damaging primary sand- and silt-sized glass shards or destroying imogolite threads in the clay fraction.

Contribution of the College of Agriculture and Home Economics Res. Ctr. Scientific Paper no. 7677.